Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
An electrical transmitter must be connected to an electrical power source to operate. One form of such a transmitter, known as a four-wire transmitter, includes two terminals for connection to a power source and two terminals for carrying an output signal proportional to the process variable. This signal can be used as an input to a controller or for purposes of indication. Because the instrument is connected directly to a power source independent from the output signal, power consumption is a less critical factor in design and use of the same.
The use of a four-wire transmitter, as discussed above, requires the use of four conductors between the transmitter and related loop control and power components. Where transmitters are remotely located, such a requirement can be undesirable owing to the significant cost of cabling. To avoid this problem, instrument manufacturers have strived to develop devices known as two-wire, or loop powered, transmitters. A two-wire transmitter includes two terminals connected to a remote power source. The transmitter loop current, drawn from the power source, is proportional to the process variable. A typical instrument operates off of a 24 volt DC source and varies the signal current in the loop between four and twenty milliamps (mA) DC. Because of these operating requirements the design of the transmitter in terms of power consumption is critical. For example, when a low level signal of four milliamps is transmitted, there is minimal power available to be consumed by the instrument. Therefore, circuits must be designed to operate off of such minimal available power.
While low power circuits are continuously developed, there are ever increasing demands placed on performance capabilities of the process control instruments. For example, with a radar level measurement device, the instrument's performance is enhanced by more powerful digital signal processing techniques driven by a microcontroller. In addition to the microcontroller, there are several other circuits, such as the radar transceiver, which require electric power. To be successful, the design must use optimum processing capability and speed. This means making maximum power from the loop available to the electronics, and making efficient use of it.
Another important concept in industrial processes is that of intrinsic safety. Much industrial equipment is installed in so-called “hazardous locations” meaning that the environment of the installation includes volatile and/or combustible material that burn or explode if exposed to an ignition source. An intrinsic-safe design, in general, requires that the circuit not have energy storage elements that can create sufficient ignition energy. This places a design restriction on the electronics; namely, the circuit design must avoid the use of large values of inductance or capacitance. If large energy storage elements are used, then the instrument must use explosion proof enclosures or encapsulation of components.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.